Method of designing blade of axial flow fluid machine and blade

ABSTRACT

To provide a method of designing a blade of an axial flow fluid machine that has a blade surface whose radius of curvature is continuous at a leading edge thereof and has a high aerodynamic performance. The method includes a step of determining a pressure surface curve and a suction surface curve as curves capable of first to third order differentiations at respective connection points to a leading edge curve, that is, a pressure surface connection point and a suction surface connection point, and a step of forming the leading edge curve as a fifth order Bezier curve that is defined by a first control point, a second control point, a third control point, a fourth control point, a fifth control point and a sixth control point. The first control point is the suction surface connection point. The sixth control point is the pressure surface connection point. Provided that an intersection of a tangent to the pressure surface curve at the pressure surface connection point and a tangent to the suction surface curve at the suction surface connection point is referred to as a point ahead of the blade, the second and fifth control points are points that internally divide line segments connecting the point ahead of the blade to the suction surface connection point and the pressure surface connection point, respectively. The third and fourth control points are points having coordinates that are determined as solutions of simultaneous equations obtained by applying continuity conditions for first to third differential coefficients at the first and sixth control points to a fifth order Bezier function.

TECHNICAL FIELD

Embodiments described herein relate to a method of designing a blade (arotor blade and a stator vane) of an axial flow fluid machine such as afan or a compressor, and to the blade.

BACKGROUND ART

The airfoil (cross-sectional shape) of a blade (a rotor blade and astator vane) of an axial flow fluid machine, such as a fan or acompressor, is formed by a leading edge, a trailing edge, and a concavepressure surface and a convex suction surface that extend between theedges.

In design of a conventional airfoil, the leading edge and the trailingedge are often shaped as a simple curve connecting the pressure surfaceand the suction surface, such as a circular or elliptic arc, whereas thepressure surface and the suction surface are specifically determined invarious methods such as those using computational fluid dynamics (CFD).

However, in the airfoil thus designed, radius of curvature of the bladesurface is discontinuous (or in other words, the curvature sharplychanges) at connections between the edges and the pressure surface andsuction surface, and the flow accelerates or decelerates at theconnections. Such a phenomenon can lead to a decrease in aerodynamicperformance of the blade, which can lead to a decrease in efficiency ora reduction of a stable operating range of the axial flow fluid machineincorporating the blade. The effect of the phenomenon is particularlymarked at the leading edge, which is located on the upstream side of theflow.

To prevent the radius of curvature of the blade surface from beingdiscontinuous as described above, various airfoil designing methods havebeen proposed (for example, Patent Documents 1 and 2).

As a technique for drawing a smooth curve, it is known to use a Beziercurve. The Bezier curve can be reshaped in various ways by displacingcontrol points number of which depends on the order of the Bezier curve.Therefore, the Bezier curve is useful for drawing a large number ofcandidate curves (for example, in a computer-aided manner) from which anoptimal curve is selected, and is widely used particularly in the fieldof industrial design.

Also in the field of design of the airfoil, attempts have been made touse the Bezier curve. For example, Patent Document 3 proposes a methodof optimizing an airfoil by drawing an existing airfoil (such as an NACAairfoil) as a Bezier curve and then reshaping the Bezier curve invarious ways.

The Bezier curve is an (N−1)-th order curve defined by N control points(N denotes the order of the Bezier curve). In the field of industrialdesign, a second or third order Bezier curve is often used. For example,a fifth order Bezier curve, which will be described later, can be drawnin the following manner (see FIG. 7).

(1) Control points X1, X2, X3, X4, X5 and X6 of the Bezier curve areselected (the fifth order Bezier curve has six control points). Thecontrol points X1 and X6 are end points of the Bezier curve.

(2) Points X7, X8, X9, X10 and X11 that internally divide the five linesegments X1X2, X2X3, X3X4, X4X5 and X5X6 obtained by connecting the sixcontrol points with a ratio of t to (1−t), respectively, are determined.

(3) Points X12 . . . that internally divide again the line segmentsconnecting the points determined in (2) with a ratio of t to (1−t) aredetermined.

(4) The same operation is repeated, and a point (X21) finally obtainedis adopted as a point on the determined Bezier curve.

(5) The fifth order Bezier curve (shown as a dotted line) is obtained byrepeating the operations (2) to (4) in the range of 0<t<1.

The Bezier curve thus drawn has a property that the line segmentconnecting an end point and the control point adjacent to the end pointis a tangent at the end point (in the example shown in FIG. 7, the linesegments X1X2 and X5X6 are tangents at the end points X1 and X6,respectively).

Therefore, in designing the airfoil, if a Bezier curve is drawn based onthe forward end points (connections to the leading edge) of the pressuresurface and the suction surface and control points on the tangents tothe pressure surface and the suction surface at these points, and theleading edge is formed by the Bezier curve, the slopes of the tangentsto the curves forming the blade surface at the connections between theleading edge and the pressure surface and the suction surface can bemade continuous (in other words, the curves are not angled at theconnections).

RELATED ART DOCUMENT Patent Document

Patent Document 1: U.S. Patent Application Publication No. 2011/0202321

Patent Document 2: U.S. Pat. No. 9,453,416

Patent Document 3: Japanese Patent Laid-Open No. 2013-3971

SUMMARY OF THE DISCLOSURE Problems to be Solved by the Disclosure

However, to solve the problems with the conventional airfoil describedabove, the continuity of only the slopes of the tangents to the curve ofthe leading edge at the connections to the pressure surface and thesuction surface is not sufficient, but continuity of also the radius ofcurvature is necessary. On the other hand, the continuity of the radiusof curvature at the connections is a necessary condition foroptimization of the aerodynamic performance of the blade but is not asufficient condition.

That is, even when the leading edge of the airfoil is formed as a Beziercurve, an enormous number of Bezier curves need to be drawn andindividually evaluated in terms of aerodynamic performance of the bladeto which the Bezier curve is applied, in order to optimize theaerodynamic performance of the blade.

However, although the Bezier curve can be reshaped in various ways bydisplacing the control points, the Bezier curve has anothercharacteristic that a desired shape cannot be obtained if the controlpoints are not appropriately selected. Therefore, it is extremelydifficult to select a Bezier curve that provides the optimal aerodynamicperformance of the blade from the Bezier curves drawn based on thecontrol points selected by trial and error.

For this reason, in forming the leading edge of the airfoil as a Beziercurve, there is a demand for a method of systematically selecting thecontrol points of the Bezier curve so that an optimal aerodynamicperformance of the blade is achieved.

The present disclosure is made in view of the problems described above,and an object of the present disclosure is to provide a method ofefficiently designing a blade of an axial flow fluid machine that has ablade surface whose radius of curvature is continuous at a leading edgethereof and has a high aerodynamic performance, and a blade of an axialflow fluid machine designed in the same method.

Means for Solving the Problems

In order to solve the problems described above, a method of designing ablade according to the present disclosure is a method to be applied to ablade having an airfoil that is formed by a leading edge curve, atrailing edge curve, a pressure surface curve and a suction surfacecurve, the method comprising:

(A) a step of determining the pressure surface curve as a curve capableof first to third order differentiations at a connection point to theleading edge curve, that is, a pressure surface connection point;

(B) a step of determining the suction surface curve as a curve capableof first to third order differentiations at a connection point to theleading edge curve, that is, a suction surface connection point; and

(C) a step of selecting a first control point, a second control point, athird control point, a fourth control point, a fifth control point and asixth control point that define a fifth order Bezier curve forming theleading edge curve, the step comprising:

(C-1) a sub-step of designating the suction surface connection point asthe first control point;

(C-2) a sub-step of designating the pressure surface connection point asthe sixth control point;

provided that an intersection of a tangent to the pressure surface curveat the pressure surface connection point and a tangent to the suctionsurface curve at the suction surface connection point is referred to asa point ahead of the blade,

(C-3) a sub-step of designating, as the second control point, a pointthat internally divides a line segment connecting the point ahead of theblade and the suction surface connection point with a ratio of p to(1−p) (where 0<p<1);

(C-4) a sub-step of designating, as the fifth control point, a pointthat internally divides a line segment connecting the point ahead of theblade and the pressure surface connection point with a ratio of q to(1−q) (where 0<q<1); and

(C-5) a sub-step of designating, as the third control point and thefourth control point, points having coordinates that are determined assolutions of simultaneous equations obtained by applying continuityconditions for first to third differential coefficients at the firstcontrol point and the sixth control point to a fifth order Bezierfunction that defines the fifth order Bezier curve.

A blade according to the present disclosure has an airfoil that isformed by a leading edge curve, a trailing edge curve, a pressuresurface curve and a suction surface curve,

the pressure surface curve is a curve that is connected to the leadingedge curve at a pressure surface connection point and whose first tothird order differential coefficients at the pressure surface connectionpoint are known,

the suction surface curve is a curve that is connected to the leadingedge curve at a suction surface connection point and whose first tothird order differential coefficients at the suction surface connectionpoint are known;

the leading edge curve is formed as a fifth order Bezier curve,

the fifth order Bezier curve is defined by a first control point, asecond control point, a third control point, a fourth control point, afifth control point and a sixth control point,

the first control point coincides with the suction surface connectionpoint,

the sixth control point coincides with the pressure surface connectionpoint,

provided that an intersection of a tangent to the pressure surface curveat the pressure surface connection point and a tangent to the suctionsurface curve at the suction surface connection point is referred to asa point ahead of the blade,

the second control point is a point that internally divides a linesegment connecting the point ahead of the blade and the suction surfaceconnection point with a ratio of p to (1−p) (where 0<p<1),

the fifth control point is a point that internally divides a linesegment connecting the point ahead of the blade and the pressure surfaceconnection point with a ratio of q to (1−q) (where 0<q<1), and the thirdcontrol point and the fourth control point are points having coordinatesthat are determined as solutions of simultaneous equations obtained byapplying continuity conditions for first to third differentialcoefficients at the first control point and the sixth control point to afifth order Bezier function that defines the fifth order Bezier curve.

Effects of the Disclosure

With the blade according to the present disclosure, since the leadingedge of the airfoil is formed as a Bezier curve, and the Bezier curve isdefined so that the first to third differential coefficients arecontinuous at the connection points to the curves forming the pressuresurface and the suction surface, the radius of curvature of the bladesurface can be made continuous at the connections of the leading edge tothe pressure surface and the suction surface, and the aerodynamicperformance of the blade can be improved.

With the blade according to the present disclosure, although the orderof the Bezier curve forming the leading edge of the airfoil is the fifthorder, by applying the continuity conditions for the first to thirddifferential coefficients described above, the number of unknownquantities is significantly reduced, and the control points of theBezier curve can be determined using only two variables. Therefore,compared with the case where control points selected by trial and errorare used, a Bezier curve having a desired shape can be quicklydetermined, and the design productivity can be improved. In addition,since the two variables remain as unknown quantities, the degree offreedom of the design of the airfoil is ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for illustrating an airfoil of a bladeaccording to an embodiment of the present disclosure;

FIG. 2 is a diagram for illustrating boundary conditions in a case wherea third order Bezier curve is applied to a leading edge of the airfoilshown in FIG. 1;

FIG. 3 is a diagram for illustrating boundary conditions in a case wherea fifth order Bezier curve is applied to a leading edge of the airfoilshown in FIG. 1;

FIG. 4A is a graph showing a shape of a part of a conventional airfoilincluding a leading edge and a distribution of the radius of curvatureof a blade surface;

FIG. 4B is a graph showing a shape of a part of the airfoil of the bladeincluding the leading edge and a distribution of the radius of curvatureof the blade surface of the present disclosure;

FIG. 5A is a graph showing a distribution of the blade surface Machnumber at a design point in the entire region in the chord direction;

FIG. 5B is a graph showing a distribution of the blade surface Machnumber at a design point in a region around the leading edge (the regionsurrounded by the dashed line in FIG. 5A);

FIG. 6 is a graph showing a loss coefficient; and

FIG. 7 is a diagram for illustrating a method of drawing a fifth orderBezier curve.

MODE FOR CARRYING OUT THE DISCLOSURE

In the following, an embodiment of the present disclosure will bedescribed in detail with reference to the drawings.

FIG. 1 is a schematic diagram for illustrating an airfoil of a bladeaccording to an embodiment of the present disclosure.

An airfoil A of a blade according to an embodiment of the presentdisclosure is formed by a leading edge LE, a trailing edge TE, and aconcave pressure surface PS and a convex suction surface SS that extendbetween the edges.

In the following, curves forming the leading edge LE, the trailing edgeTE, the pressure surface PS and the suction surface SS will be referredto as a leading edge curve CL, a trailing edge curve CT, a pressuresurface curve CP, and a suction surface curve CS, respectively, and aclosed curve formed by these four curves or, in other words, a curveforming the airfoil A will be referred to as an airfoil curve CA.

In the airfoil A of the blade according to the embodiment of the presentdisclosure, the pressure surface curve CP and the suction surface curveCS are curves capable of first to third order differentiations at apressure surface connection point TP and a suction surface connectionpoint TS both described later, respectively, and have shapesindependently determined in a process using CFD, for example. Thetrailing edge curve CT can be a circular or elliptic arc. The leadingedge curve CL is formed as a Bezier curve.

In the following, a process of determining the shape of the Bezier curveadopted as the leading edge curve CL will be described.

First, a Bezier function that defines a Bezier curve is expressed by theformula (1).

$\begin{matrix}{{{P(t)} = {\sum\limits_{i = 0}^{N}{X_{j + 1}{J_{n,i}(t)}}}}{0 = {t = 1}}} & (1)\end{matrix}$

In this formula, N denotes the order of the Bezier function, X denotesthe coordinates of a control point, and J denotes a Bernstein basisfunction defined by the formula (2).

$\begin{matrix}{{{J_{n,i}(t)} = {\begin{pmatrix}n \\i\end{pmatrix}{t^{i}\left( {1 - t} \right)}^{n - i}}}{\begin{pmatrix}n \\i\end{pmatrix} = \frac{n!}{{i!}{\left( {n - i} \right)!}}}} & (2)\end{matrix}$

The Bezier curve (leading edge curve CL) to be determined is connectedto the pressure surface curve CP and the suction surface curve CS at thepressure surface connection point TP and the suction surface connectionpoint TS, respectively. In order for the radius of curvature of theairfoil curve CA at each of these connection points to be continuous,the following boundary conditions need to be satisfied.

(1) A continuity condition for the first order differential coefficient(first order differential coefficients (dy/dx) on opposite sides of eachconnection point are equal to each other).

(2) A continuity condition for the second order differential coefficient(second order differential coefficients (d²y/dx²) on opposite sides ofeach connection point are equal to each other).

The discontinuity in radius of curvature of the conventional airfoilwhose leading edge curve is a simple curve such as a circular orelliptic arc that occurs at each of the connection point between theleading edge curve and the pressure surface curve and the connectionpoint between the leading edge curve and the suction surface curve isresolved if the boundary conditions (1) and (2) described above aresatisfied.

Furthermore, according to the boundary layer equation, it is known to bedesirable from the viewpoint of aerodynamic performance of the bladethat

(3) A continuity condition for the third order differential coefficient(third order differential coefficients (d³y/dx³) on opposite sides ofeach connection point are equal to each other) is satisfied on the bladesurface.

Thus, in determining the shape of the Bezier curve for the airfoil A ofthe blade according to the embodiment of the present disclosure, all theboundary conditions (1) to (3) described above are imposed.

First, to obtain the Bezier curve that satisfies the boundary conditionsdescribed above, what order the Bezier curve should be of will bediscussed.

Here, a third order Bezier curve has four control points, and a fifthorder Bezier curve has six control points. Two of the control points areend points of the Bezier curve, that is, the pressure surface connectionpoint TP and the suction surface connection point TS, and thecoordinates of the two control points are known. Therefore, the thirdorder Bezier curve has four unknown quantities (the x coordinate and they coordinate of the two control points other than the end points), andthe fifth order Bezier curve has eight unknown quantities (the xcoordinate and the y coordinate of the four control points other thanthe end points).

As can be seen from the above description, when a high order Beziercurve is used, a large number of unknown quantities need to be handled,and the computational complexity is high.

In view of this, first, whether the third order Bezier curve can beapplied is examined. For the third order Bezier curve, as shown in FIG.2, four control points X₁, X₂, X₃ and X₄ are used. Of these controlpoints, the control points X₁ and X₄ are the suction surface connectionpoint TS and the pressure surface connection point TP in FIG. 1,respectively.

The formula (1) can be rewritten for the third order Bezier function asfollows.

$\begin{matrix}{{P(t)} = {{\begin{pmatrix}3 \\0\end{pmatrix}X_{1}{t^{0}\left( {1 - t} \right)}^{3}} + {\begin{pmatrix}3 \\1\end{pmatrix}X_{2}{t^{1}\left( {1 - t} \right)}^{2}} + {\begin{pmatrix}3 \\2\end{pmatrix}X_{3}{t^{2}\left( {1 - t} \right)}^{1}} + {\begin{pmatrix}3 \\3\end{pmatrix}X_{4}{t^{3}\left( {1 - t} \right)}^{0}}}} & (3) \\{{P(t)} = {{{X_{1}\left( {1 - {3t} + {3t^{2}} - t^{3}} \right)} + {3{X_{2}\left( {t - {2t^{2}} + t^{3}} \right)}} + {3{X_{3}\left( {t^{2} - t^{3}} \right)}} + {X_{4}t^{3}}} = {X_{1} + {\left( {{{- 3}X_{1}} + {3X_{2}}} \right)t} + {\left( {{3X_{1}} - {6X_{2}} + {3X_{3}}} \right)t^{2}} + {\left( {{- X_{1}} + {3X_{2}} - {3X_{3}} + X_{4}} \right)t^{3}}}}} & (4)\end{matrix}$

The function P(t) is differentiated with respect to a parameter t asfollows.{dot over (P)}(t)=(−3X ₁+3X ₂)+2(3X ₁−6X ₂+3X ₃)t+3(−X ₁+3X ₂−3X ₃ +X₄)t ²  (5)

This can be rewritten as follows.

$\begin{matrix}{\frac{dy}{dx} = {\frac{\overset{.}{y}}{\overset{.}{x}} = \frac{\begin{matrix}{\left( {{{- 3}y_{1}} + {3y_{2}}} \right) + {2\left( {{3y_{1}} - {6y_{2}} + {3y_{3}}} \right)t} +} \\{3\left( {{- y_{1}} + {3y_{2}} - {3y_{3}} + y_{4}} \right)t^{2}}\end{matrix}}{\begin{matrix}{\left( {{{- 3}x_{1}} + {3x_{2}}} \right) + {2\left( {{3x_{1}} - {6x_{2}} + {3x_{3}}} \right)t} +} \\{3\left( {{- x_{1}} + {3x_{2}} - {3x_{3}} + x_{4}} \right)t^{2}}\end{matrix}}}} & (6)\end{matrix}$

At the end point X₁ (t=0) and the end point X₄ (t=1), the followingrelations hold.

dy dx ⁢ | x = x 1 = - y 1 + y 2 - x 1 + x 2 ( 7 ) dy dx ⁢ | x = x = - y3 + y 4 - x 3 + x 4 ( 8 )

The formulas (7) and (8) show that the continuity condition for thefirst order differential coefficient (boundary condition (1)) issatisfied if the control point X₂ is set on a tangent at the end pointX₁, that is, a suction surface tangent LT₁, and the control point X₃ isset on a tangent at the end point X₄, that is, a pressure surfacetangent LT₄.

By setting the control points X₂ and X₃ as described above, the numberof unknown quantities decreases from four to two. However, in order tosatisfy the continuity condition for the second order differentialcoefficient (boundary condition (2)) and the continuity condition forthe third order differential coefficient (boundary condition (3)), atleast four unknown quantities are required at this stage. This meansthat the order of the third order Bezier function is insufficient, andthe third order Bezier curve cannot be applied.

Thus, the fifth order Bezier curve is applied. For the fifth orderBezier curve, as shown in FIG. 3, six control points X₁, X₂, X₃, X₄, X₅and X₆ are used. Of these control points, the control points X₁ and X₆are the suction surface connection point TS and the pressure surfaceconnection point TP in FIG. 1, respectively.

The formula (1) can be rewritten for the fifth order Bezier function asfollows.

$\begin{matrix}{{P(t)} = {{\begin{pmatrix}5 \\0\end{pmatrix}X_{1}{t^{0}\left( {1 - t} \right)}^{5}} + {\begin{pmatrix}5 \\1\end{pmatrix}X_{2}{t^{1}\left( {1 - t} \right)}^{4}} + {\begin{pmatrix}5 \\2\end{pmatrix}X_{3}{t^{2}\left( {1 - t} \right)}^{3}} + {\begin{pmatrix}5 \\3\end{pmatrix}X_{4}{t^{3}\left( {1 - t} \right)}^{2}} + {\begin{pmatrix}5 \\4\end{pmatrix}X_{5}{t^{4}\left( {1 - t} \right)}^{1}} + {\begin{pmatrix}5 \\5\end{pmatrix}X_{6}{t^{5}\left( {1 - t} \right)}^{0}}}} & (9) \\\begin{matrix}{{P(t)} = {{X_{1}\left( {1 - {5t} + {10t^{2}} - {10t^{3}} + {5t^{4}} - t^{5}} \right)} + {5X_{2}t\left( {1 - {4t} + {6t^{2}} -} \right.}}} \\{\left. {{4t^{3}} + t^{4}} \right) + {10X_{3}{t^{2}\left( {1 - {3t} + {3t^{2}} - t^{3}} \right)}} + {10X_{4}{t^{3}\left( {1 - {3t} +} \right.}}} \\{\left. {{3t^{2}} - t^{3}} \right) + {5X_{5}{t^{4}\left( {1 - t} \right)}} + {X_{6}t^{5}}} \\{= {X_{1} + {\left( {{{- 5}X_{1}} + {5X_{2}}} \right)t} + {\left( {{10X_{1}} - {20X_{2}} + {10X_{3}}} \right)t^{2}} +}} \\{{\left( {{{- 10}X_{1}} + {30X_{2}} - {30X_{3}} + {10X_{4}}} \right)t^{3}} + \left( {{5X_{1}} - {20X_{2}} + {30X_{3}} -} \right.} \\{{\left. {{20X_{4}} + {5X_{5}}} \right)t^{4}} + {\left( {{- X_{1}} + {5X_{2}} - {10X_{3}} + {10X_{4}} - {5X_{5}} + X_{6}} \right)t^{5}}}\end{matrix} & (10)\end{matrix}$

The formulas (9) and (10) are functions that express the pressuresurface curve CP in FIG. 3. This is because the sign of the radius ofcurvature depends on how the parameter of a parametric function istaken. The suction surface curve CS can be expressed by the followingtwo formulas.

$\begin{matrix}{{P(t)} = {{\begin{pmatrix}5 \\0\end{pmatrix}X_{6}{t^{0}\left( {1 - t} \right)}^{5}} + {\begin{pmatrix}5 \\1\end{pmatrix}X_{5}{t^{1}\left( {1 - t} \right)}^{4}} + {\begin{pmatrix}5 \\2\end{pmatrix}X_{4}{t^{2}\left( {1 - t} \right)}^{3}} + {\begin{pmatrix}5 \\3\end{pmatrix}X_{3}{t^{3}\left( {1 - t} \right)}^{2}} + {\begin{pmatrix}5 \\4\end{pmatrix}X_{2}{t^{4}\left( {1 - t} \right)}^{1}} + {\begin{pmatrix}5 \\5\end{pmatrix}X_{1}{t^{5}\left( {1 - t} \right)}^{0}}}} & (11) \\\begin{matrix}{{P(t)} = {{X_{6}\left( {1 - {5t} + {10t^{2}} - {10t^{3}} + {5t^{4}} - t^{5}} \right)} + {5X_{5}{t\left( {1 - {4t} + {6t^{2}} -} \right.}}}} \\{\left. {{4t^{3}} + t^{4}} \right) + {10X_{4}{t^{2}\left( {1 - {3t} + {3t^{2}} - t^{3}} \right)}} + {10X_{3}{t^{3}\left( {1 - {3t} +} \right.}}} \\{\left. {{3t^{2}} - t^{3}} \right) + {5X_{2}{t^{4}\left( {1 - t} \right)}} + {X_{1}t^{5}}} \\{= {X_{6} + {\left( {{{- 5}X_{6}} + {5X_{5}}} \right)t} + {\left( {{10X_{6}} - {20X_{5}} + {10X_{4}}} \right)t^{2}} +}} \\{{\left( {{{- 10}X_{6}} + {30X_{5}} - {30X_{4}} + {10X_{3}}} \right)t^{3}} + \left( {{5X_{6}} - {20X_{5}} + {30X_{4}} -} \right.} \\{{\left. {{20X_{3}} + {5X_{2}}} \right)t^{4}} + {\left( {{- X_{6}} + {5X_{5}} - {10X_{4}} + {10X_{3}} - {5X_{2}} + X_{1}} \right)t^{5}}}\end{matrix} & (12)\end{matrix}$

Here, at the end point X₆, the following relation is obtained bysubstituting 1 to t in the formula (10).

$\begin{matrix}{\left. \frac{dy}{dx} \right|_{x =} = {\frac{{dy}/{dt}}{{dx}/{dt}} = {\frac{{{- 5}y_{6}} + {5y_{5}}}{{{- 5}x_{6}} + {5x_{5}}} = \frac{{- y_{6}} + y_{5}}{{- x_{6}} + x_{5}}}}} & (13)\end{matrix}$

At the end point X₁, the following relation is obtained by substituting1 to t in the formula (12).

$\begin{matrix}{\left. \frac{dy}{dx} \right|_{x = x_{1}} = {\frac{{dy}/{dt}}{{dx}/{dt}} = {\frac{{{- 5}y_{2}} + {5y_{1}}}{{{- 5}x_{2}} + {5x_{1}}} = \frac{{- y_{2}} + y_{1}}{{- x_{2}} + x_{1}}}}} & (14)\end{matrix}$

As in the case of the third order Bezier function, the formulas (13) and(14) show that the continuity condition for the first order differentialcoefficient (boundary condition (1)) is satisfied if the control pointX₂ is set on a tangent at the end point X₁, that is, a suction surfacetangent LT₁, and the control point X₅ is set on a tangent at the endpoint X₆, that is, a pressure surface tangent LT₆.

Next, the continuity condition for the second order differentialcoefficient (boundary condition (2)) will be examined.

Here, it is assumed that the suction surface tangent LT₁ and thepressure surface tangent LT₆ intersect with each other at a point X₀ahead of the blade, a point that internally divides the line segmentX₀X₁ with a ratio of (1−p) to p is designated as the control point X₂,and a point that internally divides the line segment X₀X₆ with a ratioof (1−q) to q is designated as the control point X₅ (where 0<p<1,0<q<1).

The second order differential coefficient at the end point X₁ isexpressed by the following formula.

$\begin{matrix}{\left. \frac{d^{2}y}{{dx}^{2}} \right|_{x = x_{1}} = \left. \frac{{\overset{¨}{y}\overset{.}{x}} - {\overset{.}{y}\overset{¨}{x}}}{{\overset{.}{x}}^{3}} \right|_{x = x_{1}}} & (15)\end{matrix}$

In this formula, the following relations hold.ü| _(x=x) ₁ =20x ₃−40x ₂+20x ₁  (16)ÿ| _(x=x) ₁ =20y ₃−40y ₂+20y ₁  (17){dot over (x)}| _(x=x) ₁ =−5x ₂+5x ₁  (18){dot over (y)}| _(x=x) ₁ =−5y ₂+5y ₁  (19)

However, the coordinates of the control point X₂ depends on theparameter p and therefore can be regarded as constants.

Here, substituting the formulas (16) to (19) into the formula (15)results in the following formulas.

$\begin{matrix}{\left. \frac{d^{2}y}{{dx}^{2}} \middle| {}_{x = x_{1}}{\overset{.}{x}}^{3} \right|_{x = x_{1}} = \left. {\left( {{20y_{3}} - {40y_{2}} + {20y_{1}}} \right)\overset{.}{x}} \middle| {}_{x = x_{1}}{{- \left( {{20x_{3}} - {40x_{2}} + {20x_{1}}} \right)}\overset{.}{y}} \right|_{x = x_{1}}} & (20) \\{y_{3} = {{\frac{\left. \overset{.}{y} \right|_{x = x_{1}}}{\left. \overset{.}{x} \right|_{x = x_{1}}}x_{3}} + {\frac{1}{\left. {20\overset{.}{x}} \right|_{x = x_{2}}}\left\{ \frac{d^{2}y}{{dx}^{2}} \middle| {}_{x = x_{1}}{\overset{.}{x}}^{3} \middle| {}_{x = x_{1}}{{{- 20}\left( \overset{.}{x} \middle| {}_{x = x_{1}}{y_{1} - \overset{.}{y}} \middle| {}_{x = x_{1}}x_{1} \right)} + {40\left( \overset{.}{x} \middle| {}_{x = x_{1}}{y_{2} - \overset{.}{y}} \middle| {}_{x = x_{2}}x_{2} \right)}} \right\}}}} & (21)\end{matrix}$

The formula (21) shows that, in FIG. 3, the control point X₃ must lie ona straight line LP₁ that is parallel to the suction surface tangent LT₁.

Calculating the second order differential coefficient at the end pointX₆ in the same manner results the following.

$\begin{matrix}{\left. \frac{d^{2}y}{{dx}^{2}} \middle| {}_{x = x_{6}}{\overset{.}{x}}^{3} \right|_{x = x_{6}} = \left. {\left( {{20y_{4}} - {40y_{5}} + {20y_{6}}} \right)\overset{.}{x}} \middle| {}_{x = x_{6}}{{- \left( {{20x_{4}} - {40x_{5}} + {20x_{6}}} \right)}\overset{.}{y}} \right|_{x = x_{6}}} & (22) \\{y_{4} = {{\frac{\left. \overset{.}{y} \right|_{x = x_{6}}}{\left. \overset{.}{x} \right|_{x = x_{6}}}x_{4}} + {\frac{1}{\left. {20\overset{.}{x}} \right|_{x = x_{6}}}\left\{ \frac{d^{2}y}{{dx}^{2}} \middle| {}_{x = x_{6}}{\overset{.}{x}}^{3} \middle| {}_{x = x_{6}}{{{- 20}\left( \overset{.}{x} \middle| {}_{x = x_{6}}{y_{6} - \overset{.}{y}} \middle| {}_{x = x_{6}}x_{6} \right)} + {40\left( \overset{.}{x} \middle| {}_{x = x_{6}}{y_{5} - \overset{.}{y}} \middle| {}_{x = x_{6}}x_{5} \right)}} \right\}}}} & (23)\end{matrix}$

The formula (23) shows that, in FIG. 3, the control point X₄ must lie ona straight line LP₆ that is parallel to the pressure surface tangentLT₆.

Finally, the continuity condition for the third order differentialcoefficient (boundary condition (3)) will be examined.

The third order differential coefficient is expressed by the followingformula.

$\begin{matrix}{\frac{d^{2}y}{{dx}^{2}} = \frac{{\overset{.}{x}\overset{¨}{y}} - {\overset{¨}{x}\overset{.}{y}}}{{\overset{.}{x}}^{3}}} & \; \\\begin{matrix}{\frac{d^{3}y}{{dx}^{3}} = {{\frac{d}{dt}\left( \frac{d^{2}y}{{dx}^{2}} \right)\frac{dt}{dx}} = {\frac{d}{dt}\left( \frac{{\overset{.}{x}\overset{¨}{y}} - {\overset{¨}{x}\overset{.}{y}}}{{\overset{.}{x}}^{3}} \right)\overset{.}{x}}}} \\{= \frac{\left\{ {{{\overset{.}{x}}^{2}\overset{...}{y}} - {3\overset{¨}{x}\overset{.}{x}\overset{¨}{y}} + {\left( {{3{\overset{¨}{x}}^{2}} - {\overset{¨}{x}\overset{...}{x}}} \right)\overset{.}{y}}} \right\}}{{\overset{.}{x}}^{5}}}\end{matrix} & (24)\end{matrix}$

Equations that have to be satisfied at the end point X₁ on the suctionsurface curve CS are as follows.

$\begin{matrix}{{f\left( {x_{3},x_{4}} \right)} = \left. \frac{d^{3}y}{{dx}^{3}} \middle| {}_{x = x_{1}}{- \frac{\left\{ {{x^{2}\overset{...}{y}} - {3\overset{¨}{x}\overset{.}{x}\overset{¨}{y}} + {\left( {{3{\overset{¨}{x}}^{2}} - {\overset{...}{x}\overset{¨}{x}}} \right)\overset{.}{y}}} \right\}}{{\overset{.}{x}}^{5}}} \right|_{x = x_{1}}} & (25) \\{\left. \overset{.}{x} \right|_{x = x_{1}} = {{{- 5}x_{2}} + {5x_{1}}}} & (26) \\{\left. \overset{.}{y} \right|_{x = x_{1}} = {{{- 5}y_{2}} + {5y_{1}}}} & (27) \\{\left. \overset{¨}{x} \right|_{x = x_{1}} = {{20x_{3}} - {40x_{2}} + {20x_{1}}}} & (28) \\{\left. \overset{¨}{y} \right|_{x = x_{1}} = {{20y_{3}} - {40y_{2}} + {20y_{1}}}} & (29) \\{\left. \overset{...}{x} \right|_{x = x_{1}} = {{{- 60}x_{4}} + {180x_{3}} - {180x_{2}} + {60x_{1}}}} & (30) \\{\left. \overset{...}{y} \right|_{x = x_{1}} = {{{- 60}y_{4}} + {180y_{3}} - {180y_{2}} + {60y_{1}}}} & (31)\end{matrix}$

On the other hand, equations that have to be satisfied at the end pointX₆ on the pressure surface curve CP are as follows.

$\begin{matrix}{{g\left( {x_{3},x_{4}} \right)} = \left. \frac{d^{3}y}{{dx}^{3}} \middle| {}_{x = x_{6}}{- \frac{\left\{ {{x^{2}\overset{...}{y}} - {3\overset{¨}{x}\overset{.}{x}\overset{¨}{y}} + {\left( {{3{\overset{¨}{x}}^{2}} - {\overset{...}{x}\overset{¨}{x}}} \right)\overset{.}{y}}} \right\}}{{\overset{.}{x}}^{5}}} \right|_{x = x_{6}}} & (32) \\{\left. \overset{.}{x} \right|_{x = x_{6}} = {{{- 5}x_{5}} + {5x_{6}}}} & (33) \\{\left. \overset{.}{y} \right|_{x = x_{6}} = {{{- 5}y_{5}} + {5y_{6}}}} & (34) \\{\left. \overset{¨}{x} \right|_{x = x_{6}} = {{20x_{4}} - {40x_{5}} + {20x_{6}}}} & (35) \\{\left. \overset{¨}{y} \right|_{x = x_{6}} = {{20y_{4}} - {40y_{5}} + {20y_{6}}}} & (36) \\{\left. \overset{...}{x} \right|_{x = x_{6}} = {{{- 60}x_{3}} + {180x_{4}} - {180x_{5}} + {60x_{6}}}} & (37) \\{\left. \overset{...}{y} \right|_{x = x_{6}} = {{{- 60}y_{3}} + {180y_{4}} - {180y_{5}} + {60y_{6}}}} & (38)\end{matrix}$

If the coordinates of the control point X₂ and X₅ are regarded asconstants since the coordinates depend on the parameters p and q, theremaining unknown quantities are only the X coordinates of the controlpoints X₃ and X₄, so that the simultaneous equations including theformulas (25) and (32) can be solved.

Once all the control points X₁, X₂, X₃, X₄, X₅ and X₆ are determined asdescribed above, one Bezier curve, that is, the leading edge curve CLcan be drawn in the method described earlier with reference to FIG. 7.

However, the parameters p and q are not determined yet. Therefore,Bezier curves of various shapes, that is, leading edge curves CL ofvarious shapes can be drawn by changing the parameters p and q between 0and 1. In designing the airfoil, the airfoil can be optimized byindividually evaluating the aerodynamic performance of the blades havingthe leading edge curves CL of the various shapes determined as describedabove.

Here, summarizing the method of designing the airfoil A of the bladeaccording to the embodiment of the present disclosure described above,the method includes the following steps.

(A) A step of determining a pressure surface curve CP as a curve capableof first to third order differentiations at a pressure surfaceconnection point TP.

(B) A step of determining a suction surface curve CS as a curve capableof first to third order differentiations at a suction surface connectionpoint TS.

(C) A step of selecting a first control point X₁, a second control pointX₂, a third control point X₃, a fourth control point X₄, a fifth controlpoint X₅ and a sixth control point X₆ that define a fifth order Beziercurve forming a leading edge curve CL. The step (C) includes thefollowing sub-steps.

(C-1) A sub-step of designating the suction surface connection point TSas the first control point X₁.

(C-2) A sub-step of designating the pressure surface connection point TPas the sixth control point X₆.

Provided that an intersection of a tangent to the pressure surface curveCP at the pressure surface connection point TP and a tangent to thesuction surface curve CS at the suction surface connection point TS isreferred to as a point X₀ ahead of a blade, the step (C) furtherincludes the following sub-steps.

(C-3) A sub-step of designating, as the second control point X₂, a pointthat internally divides a line segment connecting the point X₀ ahead ofthe blade and the suction surface connection point TS with a ratio of pto (1−p) (where 0<p<1).

(C-4) A sub-step of designating, as the fifth control point X₅, a pointthat internally divides a line segment connecting the point X₀ ahead ofthe blade and the pressure surface connection point TP with a ratio of qto (1-q) (where 0<q<1).

(C-5) A sub-step of designating, as the third control point X₃ and thefourth control point X₄, points having coordinates that are determinedas solutions of simultaneous equations obtained by applying continuityconditions for first to third differential coefficients at the firstcontrol point X₁ and the sixth control point X₆ to a fifth order Bezierfunction that defines the fifth order Bezier curve.

The shape of the airfoil A of the blade according to the embodiment ofthe present disclosure described above can be summarized as follows.

A pressure surface curve CP is a curve whose first to third orderdifferential coefficients at a pressure surface connection point TP areknown.

A suction surface curve CS is a curve whose first to third orderdifferential coefficients at a suction surface connection point TS areknown.

A leading edge curve CL is formed as a fifth order Bezier curve that isdefined by a first control point X₁, a second control point X₂, a thirdcontrol point X₃, a fourth control point X₄, a fifth control point X₅and a sixth control point X₆.

The first control point X₁ coincides with the suction surface connectionpoint TS.

The sixth control point X₆ coincides with the pressure surfaceconnection point TP.

Provided that an intersection of a tangent to the suction surface curveCS at the first control point X₁ and a tangent to the pressure surfacecurve CP at the sixth control point X₆ is referred to as a point X₀ahead of a blade,

the second control point X₂ is a point that internally divides a linesegment X₀X₁ with a ratio of (1−p) to p (where 0<p<1).

The fifth control point X₅ is a point that internally divides a linesegment X₀X₆ with a ratio of (1−q) to q (where 0<q<1).

The third control point X₃ and the fourth control point X₄ are pointshaving coordinates that are determined as solutions of simultaneousequations (formulas (25) and (32)) obtained by applying continuityconditions for first to third differential coefficients at the firstcontrol point X₁ and the sixth control point X₆ to a fifth order Bezierfunction that defines the fifth order Bezier curve.

As described above, with the airfoil A of the blade according to theembodiment of the present disclosure, since the Bezier curve adopted asthe leading edge curve CL is defined so that the first to third orderdifferential coefficients thereof are continuous at the pressure surfaceconnection point TP and the suction surface connection point TS, theradius of curvature of the blade surface is continuous at the connectionpoints, and the aerodynamic performance of the blade is improved.

With the airfoil A of the blade according to the embodiment of thepresent disclosure, the order of the Bezier curve adopted as the leadingedge curve CL is the fifth order. However, by applying the continuityconditions for the first to third differential coefficients describedabove, the number of unknown quantities is significantly reduced, andthe control points of the Bezier curve can be determined using only twovariables (p and q). Therefore, compared with the case where controlpoints selected by trial and error are used, that is, where eightunknown quantities (the x coordinate and the y coordinate of the fourcontrol points other than the control points at the opposite ends) arehandled, a Bezier curve having a desired shape can be quicklydetermined, and the design productivity can be improved. In addition,since the two variables (p and q) remain as unknown quantities, thedegree of freedom of the design of the airfoil is ensured.

Next, effects and advantages of the blade according to the embodiment ofthe present disclosure will be described below.

FIGS. 4A and 4B are graphs showing shapes of a part of the airfoilincluding the leading edge and distributions of the radius of curvatureof the blade surface. FIG. 4A shows a conventional airfoil, and FIG. 4Bshows the airfoil A of the blade according to the embodiment of thepresent disclosure. In each graph, the horizontal axis indicates thecoordinate x in the chord direction, the vertical axis (left) indicatesthe blade thickness y, and the vertical axis (right) indicates theradius of curvature R. FIG. 4A shows only the suction surface of theconventional airfoil, and FIG. 4B shows both the pressure surface andthe suction surface of the airfoil A.

As shown in FIG. 4A, with the conventional airfoil whose leading edgecurve CL is elliptic, the radius of curvature of the blade surface isdiscontinuous at a location (x=0) in the chord direction thatcorresponds to the connection point (suction surface connection pointTS) between the leading edge curve CL and the suction surface curve CS.

To the contrary, with the airfoil A of the blade according to theembodiment of the present disclosure, as shown in FIG. 4B, it is foundthat the radius of curvature of the blade surface is continuous at thelocations (x=0) in the chord direction that correspond to the suctionsurface connection point TS and the pressure surface connection pointTP.

FIGS. 5 and 6 show results of analysis of both the airfoils using CFDand comparison of the airfoils in terms of aerodynamic performance ofthe blade.

FIGS. 5A and 5B are graphs showing distributions of the blade surfaceMach number at a design point. FIG. 5A shows a distribution over theentire region in the chord direction, and FIG. 5B shows a distributionover a region around the leading edge (the region surrounded by thedashed line in FIG. 5A). In each graph, the horizontal axis indicatesthe distance in the chord direction (shown as non-dimensional numberswith respect to the chord length), the vertical axis indicates the bladesurface Mach number, upper plots show a distribution on the suctionsurface, and lower plots show a distribution on the pressure surface.

As shown in FIG. 5A, the distributions of the blade surface Mach numberfor the conventional airfoil and the airfoil A of the blade according tothe embodiment of the present disclosure exhibit no marked difference inthe macroscopic view over the entire region in the chord direction.However, the specific view of the region around the leading edge (theregion surrounded by the dashed line in FIG. 5A) in FIG. 5B shows that,in the distribution of the blade surface Mach number for the airfoil Aof the blade according to the embodiment of the present disclosure, theirregularities in the distribution of the blade surface Mach number forthe conventional airfoil are eliminated, and the blade surface Machnumber changes smoothly.

That is, with the airfoil A of the blade according to the embodiment ofthe present disclosure, acceleration and deceleration of the flow overthe blade surface are reduced compared with the conventional airfoil,and the fan or compressor incorporating the blades with the airfoil A isexpected to have a higher efficiency and a wider stable operating range.

FIG. 6 is a graph showing a loss coefficient. The vertical axisindicates the loss coefficient (a non-dimensional index of a totalpressure drop at the time of passing over the blade with respect to adynamic pressure), and the horizontal axis indicates the flowrate (theamount of intake air of the fan or compressor incorporating the blades).The flowrate corresponds to the incidence (inflow angle) at the inlet ofthe blade. The incidence is low when the flowrate is high, and is highwhen the flowrate is low.

As shown in the drawing, it is found that when the flowrate is high (theincidence is low), no marked difference is observed. However, when theflowrate is low (the incidence is high), the airfoil A of the bladeaccording to the embodiment of the present disclosure provides smallerloss coefficients than the conventional airfoil.

With the conventional airfoil, the loss coefficient is high when theflowrate is low (the incidence is high), and in order to preventoccurrence of a stall, the operating range needs to be limited to arange of higher flowrates (lower incidences).

To the contrary, with the airfoil A of the blade according to theembodiment of the present disclosure, the loss coefficient is low evenwhen the flowrate is low (the incidence is high), so that the operatingrange does not need to be limited to a range of higher flowrates (lowerincidences). That is, the fan or compressor incorporating the bladesaccording to the embodiment of the present disclosure can have a stableoperating range expanded to lower flowrates compared with the fan orcompressor incorporating the blades with the conventional airfoil.

Although the airfoil only the leading edge curve of which is formed as aBezier curve has been described above, not only the leading edge curvebut also the trailing edge curve can be formed as a Bezier curve.

Although the blade according to the present disclosure is particularlyadvantageous when the blade is used as a rotor blade or stator vane of afan or compressor of a turbo-fan engine of an aircraft which has a highblade surface Mach number and experiences a significant change inincidence (inflow angle) during operation, the application of the bladeis not limited thereto. The blade according to the present disclosurecan be widely used as a rotor blade or stator vane of an axial flowfluid machine, such as a compressor of a gas turbine other than theturbo-fan engine for an aircraft, or a fan, a compressor or a turbine asa stand-alone device.

EXPLANATION OF REFERENCE SIGNS

-   -   A airfoil    -   LE leading edge    -   CL leading edge curve    -   TE trailing edge    -   CT trailing edge curve    -   PS pressure surface    -   CP pressure surface curve    -   SS suction surface    -   CS suction surface curve    -   TP pressure surface connection point    -   TS suction surface connection point    -   X0 point ahead of blade    -   X1 first control point    -   X2 second control point    -   X3 third control point    -   X4 fourth control point    -   X5 fifth control point    -   X6 sixth control point

The invention claimed is:
 1. A blade having an airfoil that is formed bya leading edge curve, a trailing edge curve, a pressure surface curveand a suction surface curve, wherein the pressure surface curve is acurve that is connected to the leading edge curve at a pressure surfaceconnection point and whose first to third order differentialcoefficients at the pressure surface connection point are known, thesuction surface curve is a curve that is connected to the leading edgecurve at a suction surface connection point and whose first to thirdorder differential coefficients at the suction surface connection pointare known; the leading edge curve is formed as a fifth order Beziercurve, the fifth order Bezier curve is defined by a first control point,a second control point, a third control point, a fourth control point, afifth control point and a sixth control point, the first control pointcoincides with the suction surface connection point, the sixth controlpoint coincides with the pressure surface connection point, providedthat an intersection of a tangent to the pressure surface curve at thepressure surface connection point and a tangent to the suction surfacecurve at the suction surface connection point is referred to as a pointahead of the blade, the second control point is a point that internallydivides a line segment connecting the point ahead of the blade and thesuction surface connection point with a ratio of p to (1−p) (where0<p<1), the fifth control point is a point that internally divides aline segment connecting the point ahead of the blade and the pressuresurface connection point with a ratio of q to (1−q) (where 0<q<1), andthe third control point and the fourth control point are points havingcoordinates that are determined as solutions of simultaneous equationsobtained by applying continuity conditions for first to thirddifferential coefficients at the first control point and the sixthcontrol point to a fifth order Bezier function that defines the fifthorder Bezier curve.